### The physics of flying seeds

**Purpose: **Students will design, build, test and evaluate model fliers with the goal of making the fliers travel as far as possible.

**Procedural overview: **After reading the *Science News* article “Whirling maple seeds inspired these tiny flying sensors,” students will outline an engineering problem and develop criteria and constraints for possible solutions. Student groups will then design, build and test models of fliers. After initial testing to optimize their models to travel as far as possible, groups will test their final models. Together, the class will compare the performance of all the models to identify the characteristics of the best-performing design and relate those characteristics to physical science principles.

**Approximate class time: **2–3 class periods

**Supplies:**

Photographs or samples of seeds that are dispersed by wind

Objects that can be used to model seeds, such as small beads, tiles or bearings; coins; washers; guitar picks; or actual seeds

Materials that can be used to model wings or other seed structures, such as pipe cleaners, paper clips, straws, tissue paper, plastic film, streamers, cotton balls, yarn or string

Materials for adhering or joining, such as glue, tape, thread or string

Scissors or craft knives

Measuring tape

Stopwatch

Ring stand or PVC pipes and fittings for the apparatus from which models will be dropped

A box fan or desktop fan (optional)

Protective eyewear

Computer with internet access

Interactive meeting application for virtual learning (optional)

Audio or video capture and editing hardware and software (optional)

**Want to make it a virtual lesson?** Group discussions can be conducted using interactive meeting software and video-sharing technologies. Student groups should use screen-sharing and file-sharing applications in which documents can be simultaneously edited in real time.

Students should be able to construct models individually at home, and some testing for how well models perform when dropped can be done at home. Testing and comparing of models is best done in the classroom where space and equipment are available. Testing can be done by the teacher, and the results can be shared with students by video.

**Directions for teachers:**

**The setup**

For homework before the first class, have students read the online *Science News* article “Whirling maple seeds inspired these tiny flying sensors” and answer the following questions. A version of the article, “Tiny fliers take after maple seeds,” appears in the November 6, 2021 issue of *Science News*. Please note that the provided answers are examples and that student answers will vary.

1. What characteristics of maple seeds in flight inspired engineers to make the microfliers?

*Engineers were inspired by the way maple seeds whirl or helicopter gracefully as they glide.*

2. What benefits do the shapes of the microfliers provide?

*The wings or blades cause spinning motions, which stabilize the microfliers’ flight and decrease the speed at which the object falls. This reduced speed enables the microfliers more time in the air so that they can travel farther.*

3. What are some of the uses scientists envision for these microfliers?

*Scientists think these microfliers will have many uses, including carrying sensors to measure and monitor environmental conditions, testing for heavy metals, assessing hazards like chemical spills or tracking conditions in the atmosphere.*

4. Research the structure of a maple seed, or samara. Draw a diagram of the maple seed and label the following structures and dimensions: seed, area of concentrated mass, center of mass, base tip, wingtip, wing, leading-edge, maximum wing width (wing chord) and length of seed (span).

*Student diagrams will vary. For example diagrams, see Figure 2 in M.H. Sohn et al, 2019 and Figure 7 in M.Y. Zakaria et al, 2019.*

5. Why do maple seeds spin as they fall?

*Maple seeds consist of a heavy seed attached to a papery or membranous wing. The seed’s center of mass is located on the boundary where the seed is attached to the wing membrane. As the seed detaches from the tree, the seed side, where the center of mass is located, falls toward the ground at a faster rate than the less dense wing side of the seed. With the seed falling with the seed part down and the wing up, the variations in the wing interrupt the laminar flow of air, causing turbulence. This turbulence causes side-to-side motion of the wing, which accelerates to form a spinning vortex of air along the front leading edge of the seed. This vortex lowers the air pressure over the seed. In response to the lower air pressure, the wing lifts upward, opposing gravity and increasing air resistance. In other words, the wing acts as an airfoil, which causes an angular rotation of the seed. This angular rotation generates lift along the underside of the wing, which slows the rate at which the seed falls.*

6. Study the photograph of the microfliers in the article. How would you describe the shapes of the microfliers? What do the microfliers have in common with each other and with the maple seed you drew?

*The microfliers are three-dimensional and have low-profile shapes. Most have three blades or wings arranged in a triangle. Most of the microfliers’ centers are higher or lower than the wings or blades. Some of the microfliers appear to have wide, single blades or wings connected by a hinge to a shaft. These microfliers mimic the maple seed because they use wings, blades or sails to induce spinning motions.*

**Class period 1**

**Defining the design problem**

As a class, students will define an engineering problem and establish criteria for the solution to be successful as well as constraints for the solution. Tell students that they will be designing models inspired by flying maple seeds to determine which combinations of structures may be most beneficial for dispersing seeds. Have students discuss and answer the following questions as a class. Please note that the provided answers are examples and that student answers will vary.

If time is short, provide student groups with materials for building their models and a list of criteria and constraints that you have developed for them. Have students work quickly through the brainstorming and initial design stages and begin constructing their first model. To save time, reduce the number of iterations each group completes before moving to the final competition and analysis.

1. Define the engineering problem that needs to be solved.

*The engineering problem we need to solve is how to build a flier that will travel the farthest from a starting point using structures like those in plant seeds.*

2. What are the requirements that the model needs to meet to be successful?

*The main criterion is that the final model must carry a “seed” at least 4 meters and ideally as far from the starting point as possible as measured by the horizontal distance on the ground. The model must incorporate structures like those used by wind-dispersed plant seeds.*

3. What are the limitations or constraints of the model?

*The model must be built using materials available in the classroom. The model must carry a “seed” that has a mass of 2.5 grams and be 20–120 millimeters long.*

4. How has the problem already been solved by plants?

*Plants have evolved seed structures like membranous wings or parachutes that use wind, air resistance and vortexes to slow their seeds’ descent so that the seeds stay in the air longer and travel farther.*

**Develop possible solutions**

After the class has defined the problem and established criteria and constraints for design solutions, organize the class into groups. Each group will develop and refine a design for a flier. Groups will then construct models of their designs that will compete at the beginning of the next class period.

As students design their fliers, have them use the following questions to guide their development. Please note that the provided answers are examples and that student answers will vary.

1. What scientific principles are going to guide your design?

*We are going to focus on increasing air resistance to slow the velocity at which the flier falls. We are also going to use Bernoulli’s principle to create lift that will cause the flier to rotate.*

2. What features could you build into your flier? Brainstorm ideas for how to construct a flier that meets the criteria and constraints determined by the class. Record all ideas as they are presented and build on the ideas of others.

*1) Build a flier that is shaped like a maple seed. **2) Build a flier that is shaped like a maple seed but has two or three straight wings that extend from the same side of the seed. **3) Build a flier that is like a maple seed but has two or three bent wings that extend from the same side of the seed. **4) Build a flier that has wings on the opposite side of the seed. **5) Build a flat flier that is shaped like a disc with the seed at the center. **6) Build a flier that is shaped like a shuttlecock. **7) Build a flier that is shaped like a dandelion seed. **8) Build a flier that has a parachute that is pulled open as the seed falls off the branch.*

3. Which design idea has the most potential for success? Use the criteria and constraints developed by the class and the research about how seeds glide to compare ideas.

*You may want to introduce* *students to the concept of a decision matrix. A decision matrix lists all the criteria and assigns a numerical ranking to each criterion based on how essential that criterion is to the success of the design. Students can evaluate each proposed solution and assign that solution a numerical ranking for each criterion based on how well the proposed solution meets that criterion. The numbers are added up, and the solution that has the highest ranking is determined to be the best fit. Decision matrices can be constructed as tables or in spreadsheets. Several free and open-source decision matrix templates for Excel can be found online, such as one from **Someka Excel Solutions**.*

*Numbers in column heads represent the proposed solutions listed in the sample answer to question No. 2 above.*

| | Ratings | |||||||

Criterion | Weight | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

It must carry a seed as far from the starting point as possible as measured by the horizontal distance on the ground, but must travel at least 4 meters. | 5 | | | | | | | | |

It must incorporate structures like those used by wind-dispersed plant seeds. | 4 | | | | | | | | |

It must be built using the materials available in the classroom today. | 3 | | | | | | | | |

It must carry a “seed” that has a mass of 2.5 grams. | 4 | | | | | | | | |

It must be at least 20 millimeters long and cannot be larger than 12 centimeters long. | 3 | | | | | | | | |

4. Select one solution for your group to model. What materials do you need? How will you join the components together?

*We chose to build a flier that is shaped like a maple seed but has three straight wings that extend from the same side of the seed. We will build it using a coin as a stand-in for the seed, paper clips as the stiff leading edge of the wings and tissue paper as the wing membranes. We will bend the paper clips so that they are straight and then use the paper clips to make a cage to hold the coin. The ends of the paper clips will extend upward from the top of the cage. We will cover each wing with tissue paper that is glued together to make a double layer membrane. Each wing will be 6 cenimeters long, and the entire flier will be 9 cm long.*

**Class period 2**

**Test and refine your solution**

Have student groups construct a model of their designs using the provided materials and following the criteria and constraints established by the class. Advise students to use caution when using scissors or utility knives, when joining segments of their models and when testing their models. Require students to wear goggles or other eye protection during this stage of the activity. Please note that the provided answers are examples and that student answers will vary.

1. Construct a model of your design. Record the materials you used, the mass, length, diameter, angles of the wings and other relevant details of your design.

*Encourage students to draw diagrams and construct tables to display and record relevant information.*

2. Test your model by releasing it from a selected height. Time how long the flier falls between when it is dropped and when it touches the floor. Measure the distance the flier travels from the point on the floor directly beneath the point of release. Perform the drop three times, and record the results of each trial.

*Drop 1: 3 meters, 5.4 seconds; Drop 2: 2.7 m, 4.9 s; Drop 3: 3.2 m; 6 s*

3. Review the results of your test. How well did your model meet the criteria?

*The model carried the seed horizontally, but it did not travel at least 4 m from the starting point. So the design solution did not meet a primary criterion.*

4. Discuss how you could make improvements to your design. Try asking the following questions as you analyze your design.

What about the design worked?

Why was that element successful?

What function did the element serve?

Can you improve on that element in your design?

What about the design didn’t work?

What function did that element serve?

Why wasn’t that element successful?

How did that element detract from the overall design?

Can you improve on that element in your design?

How could you modify the existing design to improve your model’s performance?

*The wings made the flier rotate as it fell, which slowed down the rate at which the seed fell. The overall mass of the flier is too high, so the wings couldn’t slow the flier down enough for it to travel far horizontally. We could change the number of wings, the materials the wings are made of or the angle at which the wings extend from the top of the seed.*

5. Choose one element to change in your design. Describe the change you will make and why you chose that element to change. Build a new model using your revised design.

*We are going to change the angle at which the wings extend from the top of the seed. We think that by making the wings protrude more to the sides rather than straight back, we can increase the rate of spin and slow the flier’s descent.*

6. Test your new model. Record how the new model performed.

*Drop 1: 3.5 m; 6.6 s; Drop 2: 3.7 m; 6.9 s; Drop 3: 3.6 m; 6.7 s*

7. How did the change you made to your design affected your model’s performance? Why do you think you got that result?

*The new model took longer to reach the floor and traveled farther horizontally than the previous model did. Changing the angle of the wings slowed down the rate of descent, which increased the distance the flier traveled outward from the drop point. However, the flier still did not travel at least 4 m from the starting point, so it does not meet the primary criterion and must be redesigned.*

8. Discuss how you could make improvements to your design. Ask the same questions you used the first time you analyzed your design. Will you build a modified model based on your first design or your second design?

*We need to slow down the rate of the descent even more so that the flier has time to travel outward at least 4 m. Because the second model performed better than the first, we should modify the second model. We should try bending the wings so that they point straight back from the seed but then bend outward at a 90**°** angle.*

9. Construct and test a new model based on your design review. How did the new model perform?

*Drop 1: 4.0 m, 7.2 s; Drop 2: 4.1 m, 7.4 s; Drop 3: 4.0 m, 7.3 s; This design met the primary criterion of traveling at least 4 m from the starting point. The model stayed in the air longer than the others, which allowed it time to travel farther.*

10. Repeat the testing and redesigning process as many times as needed within the class period until you have optimized your design. How many models did you build? Record all the changes to the design as you make them.

*We made two more models. In the first one, we changed the angle of the wing chord to be horizontal. It did not perform as well as the previous model, so we changed the angle of the wing chord to be at 45**°**. Our fifth model has performed the best so far, with an average falling time of 10 s and an average horizontal distance traveled of 5.5 m.*

11. Construct your final model and store it as directed by your teacher. This is the model you will use to compete with the other fliers constructed by other groups.

*Students should construct a final model before the end of class so that whatever adhesives used can set overnight before testing. Make sure students have a safe place to store their models where the fliers will not be disturbed or damaged before the next class period.*

**Class period 3**

Before class begins, set up the drop apparatus so that the drop height can be maintained for all trials. The drop height should be at least 2 m, and drop heights of 3 or 4 m are better.

If you want to simulate wind conditions in an enclosed area, set a location for a box fan that will not be blocked by observers or participants. The apparatus itself can vary depending on the resources available. A scaffold structure composed of a ring stand and a hinge or trap that can consistently drop models from a given height is one option. Another option includes hanging models by threads from a bar of a set height and cutting the thread to begin the drop.

**Check and refine your final design**

At the beginning of the second class period, groups should prepare to test their models against those of other groups. Students will have the chance to finalize their models and make any final upgrades. Then, students will test all the models under the same conditions to compare the success of the different designs. Please note that the provided answers are examples and that student answers will vary.

1. Review your model. Compare it to the criteria and constraints. Is your model complete and ready to test? Describe any adjustments you need to make to your design to prepare your model for testing.

*We checked our model and it did not need any repairs or changes.*

2. Make any necessary changes to your design. Record the materials you used, the mass, length, diameter, angles of the wings and other relevant details of your final design.

*We did not make any changes to our design. The mass of the model is 5 g, of which 2.5 g is the coin that represents the seed. It is 8.5 cm in diameter at the widest point, and it is 4 cm long. In testing, it traveled farther than 4 m horizontally. It is composed of a coin, six paperclips, tissue paper and glue. The wings attach to the end of the cage that contains the seed, extend 2 cm back and then bend 90**°**, perpendicular to the length of the flier. The wing chords are set at a 45**°** angle upward from the angle of the leading edges of the wings.*

**Test and compare models**

Before class begins, make sure the testing apparatus is set up. The testing apparatus should consist of an anchor point located where group models can be hung from a string at the same height. The string can be cut to start each trial, to ensure that each trial starts from the same position and speed.

As a class, test and compare the distance traveled by the group models. Students should record the data from each test and use it to compare the models. The class will declare a “winning” design that best meets the criteria and constraints developed by the class. Please note that the provided answers are examples and that student answers will vary.

1. Construct a data table to record the information about the group models. Make sure you include information about the models that will allow you to compare their designs after the testing is completed.

*Sample table:*

Model | Description & diagram | Mass (g) | Length (cm) | Diameter (cm) | Position of center of mass | Number of wings | Angle of wings |

Group A | Shaped like a shuttlecock with bent wings | 5 | 4 | 8.5 | ¼ of the length; 1 cm from the base tip of the seed | 4 | 45 |

Group B | | | | | | | |

Group C | | | | | | | |

2. Each group should test its model three times following the procedure described by your teacher. Record the results of each trial in a data table that records the trial number, the distance traveled and the falling time of each model.

Model | Trial | Distance (m) | Falling time (s) |

Group A | 1 | ||

2 | |||

3 | |||

Group B | 1 | ||

2 | |||

3 | |||

Group C | 1 | ||

2 | |||

3 |

**Analyze the results**

After testing is complete, students should analyze the results to identify a “winning” design. Students should analyze the results of the test as a class and discuss the answers to the following questions. Please note that the provided answers are examples and that student answers will vary.

1. Analyze the data you recorded for each trial. Which model performed the best? How did you determine this model performed the best?

*The model that was shaped like a shuttlecock with bent wings performed the best because its average distance traveled horizontally was the greatest of all the models.*

2. What structures optimized the performance of the best model?

*The model had a concentrated mass at one end and several rigid, membranous wings extending away from the concentrated mass. The wings were attached at an angle to the center of mass. The wings were rigid but lightweight.*

3. In what way could the quantitative data be displayed visually to correlate the models’ structures with distance traveled?

*The data could be displayed as a bar chart or a histogram that plots each design by the distance traveled. The data could also be plotted as a scatterplot that compares the distance traveled to wing length or width, for instance.*

4. Imagine that you could influence the shape of maple seeds through genetic engineering. What seed structures or features would you modify to increase the distance that it can be dispersed by wind?

*I would want to change the angle at which the wings attach to the seed and increase the number of wings, so that the maple seeds were structured more like the flier that traveled the farthest in our test.*

**Possible extension:** If there is time or if students are interested in pursuing an independent research project, have students think about their local environment, including weather conditions, plant populations, animal populations and geologic formations. Ask students to consider how seeds spread in this environment. Ask them to imagine what it would take to reseed a native species that was once widely distributed around the region. If seeds for this plant were available, how could students use what they have learned in this activity to design a seed packet to disperse the seeds?

Students can present their responses as sketches or engineering diagrams for their proposed seed packet designs.

**Additional resources**

*Science News* articles:

R. Ehrenberg. “How maple fruits fall.” *Science News*. Published online June 11, 2009.

Scientific articles:

B.H. Kim *et al*. Three-dimensional electronic microfliers inspired by wind-dispersed seeds. *Nature*. Vol. 597, September 23, 2021, p. 503. doi: 10.1038/s41586-021-03847-y.

M.Y. Zakaria *et al*. Modeling and prediction of aerodynamic characteristics of free fall rotating wing based on experiments. IOP Conference Series: Materials Science and Engineering. **610** (2019) 012098. doi: 10.1088/1757-899X/610/1/012098.

K. Varshney *et al*. The kinematics of falling maple seeds and the initial transition to a helical motion. *Nonlinearity*. Vol. 25, December 15, 2011, pp. C1–C8. doi: 10.1088/0951-7715/25/1/C1.

Other resources

“Gone with the Wind: Plant Seed Dispersal.” Science Buddies.